As emission requirements continue to become more stringent, engine manufacturers and component suppliers continue to improve engine operation. One area that has received particular focus has been fuel injection. By more accurately controlling fuel injection, improved combustion can be achieved, providing better engine efficiency and reduced emissions.
One type of fuel injector that has received much attention has been the common rail injector. The common rail fuel injector controls the injection of high-pressure fuel that the injector receives from a high-pressure fuel rail. The injector does not pressurize the fuel but simply controls injection by controlling the check valve. Typically, high-pressure fuel is constantly present in the tip of the fuel injector and injection occurs by actuating a control valve to vent a check control cavity, allowing the high-pressure fuel in the tip to push the check valve up.
Although the common rail injector provides good control of fuel injection, improvement is still necessary. Specifically, the common rail injector has limited rate-shaping capability, generally a square rate shape, due to the fact that high-pressure fuel is always present in the tip. Further, the common rail fuel injector's delivery curve is not linear and can have unusable ranges because fuel injection starts as soon as the control valve is actuated, as opposed to waiting until the control valve is seated.
Furthermore, leakage of high-pressure fuel within the injector contributes to losses and less than optimal system efficiency, as such leakage requires the pump to pressurize such fuel, yet the system does not benefit from the fuel which leaks.
The constant presence of high-pressure fuel in the tip of such common rail injectors is also seen as a potential source of engine damage, should the nozzle needle remain in an open or partially-open position. One way to address this concern is changing the internal plumbing arrangement of the injector's valves and lines to form an admission valve. Such admission valves only allow high pressure fuel to be present in the tip only when injection is desired, rather these valves block the high pressure from reaching the tip during the non-injection period and vent any pressure remaining in the tip at the end of injection back to tank. Typical common rail injectors in production today utilize a 3-port, 2-position valve, and do not block the fuel from reaching the tip during the non-injection period.
Some admission valves are described as a control slide, or spool valves, whose control edges meter the fuel quantity to be delivered, and even attempt to limit leakage losses by closing the outlet side opening before opening the inlet side opening. However such spool valves must have diametral clearance to move, and such clearance forms a leakage path that contributes to losses.
An exemplary admission valve is shown in U.S. Pat. No. 5,538,187. This admission valve improves the control valve by forming a poppet valve rather than a spool valve. Such valves are known to seal better than spool valves, and therefore have lower leakage losses. The other end forms a flat valve seat, which are known to be difficult to achieve a tight seal, versus that possible with a poppet valve.
The present application addresses one or more of the problems identified above.